The present invention relates to a fixing device, and more particularly relates to a fixing device for fixing images on a sheet with use of heat from a fixing roller heated by the electromagnetic induction heating method.
A fixing device of this kind has been known as described in JP 2000-214702 A or JP 2000-214713 A. The fixing device has a fixing roller and a pressure roller in pressure-contact with each other, wherein an electromagnetic induction heat generating layer (hereinbelow referred to as “heat generating layer”) of the fixing roller is heated by a magnetic flux generated in a magnetic flux generating section. Then, a recording member carrying an unfixed image is held and transported by a nip section, which is made up of a pressure-contacted portion of the rollers, so as to melt and fix the unfixed image on the recording member. To enhance a temperature rise characteristic by reducing thermal capacity, a thin nickel-electroformed endless belt layer of e.g. 100 μm in thickness is used for the heat generating layer of the fixing roller.
As shown in an equivalent circuit of FIG. 12, an electric power to the magnetic flux generating section is conventionally supplied by a high frequency (HF) inverter 104 including a parallel resonant circuit 142. The HF inverter 104 includes an AC power source 140, a rectification circuit 141 made up of a diode bridge DB141, a smoothing coil Lf141 and a smoothing capacitor Cf141, a switching element 145 made from a power transistor, a flywheel diode D145 for protecting the switching element 145 from overvoltage, and the parallel resonant circuit 142 including a resonant capacitor 144. The resonant capacitor 144 is connected in parallel to a coil 143 (placed along the fixing roller) included in the magnetic flux generating section. An inductance and an effective resistance (including contribution from the fixing roller coupled with the coil by electromagnetic induction) observed on both ends of the coil 143 are respectively referred to as Ls143 and Rs143.
In the case where the heat generating layer (nickel layer) of the fixing roller has a thickness as thin as 100 μm or less, high heat generation efficiency can be attained by driving the fixing roller at higher frequencies to decrease a depth (unit: m) of penetration as shown by Equation (1).Depth of penetration=1/(Πfμρ)1/2  (1)where f represents a drive frequency (unit: Hz), μrepresents magnetic permeability of the heat generating layer (unit: H/m) and ρ represents conductivity of the heat generating layer (unit: S/m).
In the case where the heat generating layer (nickel layer) has a thickness of 40 μm for example, the drive frequency f is required to be at least about 40 kHz and ideally be 60 kHz or more.
As is clear from drive waveforms in FIG. 13, input power (which depends on a current ILs flowing through the coil 143) is dependent on a length of a turn-on period T (which is a period when a collector-emitter voltage VCE is low) of the switching element in the HF inverter 104. If the drive frequency f is made higher, then the turn-on period T of the switching element is shortened, which makes it difficult to secure high input power. For example, for securing power of about 1200 W, an upper limit of the drive frequency f is about 25 kHz˜30 kHz.